Since working environments are dramatically changing, it is difficult to support collaboration among people. In this talk, I will claim that the context should be efficiently synchronized. To efficiently support collaborations between people (agents) in real-time, we propose an ontology-based platform for acquainting the most relevant users (e.g., colleagues and classmates), according to their context. Thereby, we have modeled two kinds of contexts with semantic information derived from ontologies; (i) personal context, and (ii) consensual context, integrated from several personal contexts. More importantly, we formulate measurement criteria to compare them. Consequently, groups can be dynamically organized with respect to the similarities among several aspects of personal context. In particular, users can engage in complex collaborations related to multiple semantics. For experimentation, a social browsing system has been implemented based on context synchronization.